1. Field of the Invention
The present invention relates generally to a target structure for use with an image pick-up tube, and is directed more particularly to a target structure for use with an image pick-up tube which is simple in construction but effectively avoids deterioration of characteristics.
2. Description of the Prior Art
In the art, an amorphous selenium Se photo-conductive layer has been known as the photo-conductive layer of a target for use with an image pick-up tube. This Se photo-conductive layer is small in lag, so that it is employed widely as the photo-conductive layer of the target for the image pick-up tube. In order to increase the sensitivity for a red color (long wavelength), tellurium Te is added to the Se photo-conductive layer. However, when Te is added to the Se photo-conductive layer, its heat-resisting property becomes low, and especially such a deterioration of characteristics appears that a dark current increases by aging, sensitivity is varied, twinkling is caused and so on.
In order to remove the above defect arsenic As is further added to the Se photo-conductive layer. In this case, if the concentration of As is merely increased, the variation of sensitivity of the layer becomes great upon continuous operation.
To avoid this latter defect, such a target structure using a Se-Te-As photo-conductive layer shown in FIG. 1 is proposed for an image pick-up tube. This target structure consists of a transparent conductive layer 2 made of, for example, SnO.sub.2, which is coated on the inner surface of a glass face plate 1 of an envelope of the image pick-up tube, for deriving a signal current, a stabilizing layer i.e. priming layer 3 made of, for example, ZnO or GeO.sub.2 coated on the transparent conductive layer 2, a Se-Te-As photo-conductive layer 4 coated on the priming layer 3, and a so-called beam-landing layer 5 made of, for example, antimony trisulfide Sb.sub.2 O.sub.3 coated on the photo-conductive layer 4. In this case, an electron beam impinges on the layer 5 along the direction indicated by an arrow a in the figure.
The Se-Te-As photo-conductive layer 4 consists of a first protective layer 6, a sensitized layer 7, a second protective layer 8 and a capacitive layer 9 for reducing the electrostatic capacity of the target in this order as shown in FIG. 1. The capacitive layer 9 is made of Se-As photo-conductive layer whose As concentration is lower than, for example 5 atom% and whose thickness is selected sufficiently great, for example, 4 .mu.m(micron). Each of the first and second protective layers 6 and 8 is made of, for example, a Se-As photo-conductive layer which contains As at relatively high concentration, and the sensitized layer 7 is made of a Se-Te-As photo-conductive layer which contains Te of, for example, 20 atom%.
An image pick-up tube device, which is provided with the above target structure, has less deterioration so that dark current increases upon continuous operation and the sensitivity for the red color is varied upon high temperature (.about.50.degree. C.) operation and during reservation. It is considered that this advantage is caused by the fact that the diffusion to Te in the sensitized layer 7 is prevented by the existence of the first and second protective layers 6 and 8.
The above target structure is, however, complicated in construction and hence rather expensive.